Printing is generally divided into certain basic types including what may be generally termed letterpress, offset and flexographic. Each of these types have been adapted to rotary printing presses and letterpress and offset are commonly employed in large and small printing press installations including newspaper printing presses. Letterpress printing provides for the transfer of an impression from the raised surfaces of printing plates to paper passed between a plate cylinder carrying such plates and a resiliently covered impression cylinder. On the other hand, offset printing provides for the transfer or offset of prints from a plate by the utilization of an intermediate offset cylinder or blanket cylinder. Modern offset printing presses normally print blanket-to-blanket, i.e., employ the blanket cylinder for one side of the sheet as the impression cylinder for the other so that printing is accomplished on both sides of the sheet simultaneously. As an intermediate to letterpress and offset there has been developed planographic printing which employs a smooth surfaced plate, chemically treated so that the portion of the plate that is to do the printing is the only portion to which printing ink will adhere. The printing takes place directly on the sheet of paper. The printing surface of a planographic plate is on the order of an offset plate with the exception that the typematter is set in reverse. offset plates and planographic plates are on the order of 7 to 30 thousandths of an inch in thickness and are flexible.
Many press installations, particularly in the field of rotary printing presses for newspapers, are designed for the use of stereotype letterpress printing plates. Oftentimes, however, it is desired to convert from letterpress to offset printing particularly in the field of newspaper printing. This is a major operation requiring the construction and fabrication of complete new printing presses which is not only costly but also time consuming. Dummy mounting saddles are presently available on which to attach thin offset or planographic printing plates, thus plate cylinders do not necessarily have to be changed when methods of printing are changed.
Although many of the elements of a rotary stereotype printing press may be employed in an offset press, it has not heretofore been possible to convert one type of press to the other. In an offset press the combined impression and offset cylinders are mounted in a different position and thus it has been necessary to employ different press side frames for different types of printing. This then requires the installation of completely new presses. It is furthermore noted that the thickness of printing plates employed in letterpress and offset printing are markedly different and consequently a plate cylinder that is designed for the use of stereotype printing plates must be made adaptable for the use of offset printing plates or planographic printing plates. A plate cylinder designed for the use of stereotype plates is usable in the present press but a change of cylinders is recommended if the press is to be permanently converted from letterpress to offset. The change can however, be made "in the field" without press alteration other than the change of said cylinders. If plate cylinders designed for the use of stereotype printing plates are to be retained, then it will be necessary to employ dummy mounting plates when the offset or planographic printing method is employed.
While it would be highly advantageous to be able to progressively convert individual press units of a multible unit installation from stereotype letterpress to offset printing, such has been previously precluded by the above-noted difficulties. The present invention provides for such a successive conversion.